The present invention relates to a device for controlling the water balance of patients undergoing haemodialysis. Haemodialysis treatment is known to consist in feeding the patient's blood into a dialyser having a semipermeable membrane, along a first face of which is fed the patient's blood, and along a second face of which is fed a dialysing fluid. Between the said two membrane faces, substances are exchanged enabling the elimination of metabolic waste and restoration of the electrolytic balance in the blood. During such treatment, steps must also be taken to restore the patient's water balance, by drawing off the water accumulated since the last treatment. To do this, a positive pressure is produced on the blood side of the membrane, and a negative pressure on the dialysing fluid side. The difference between the two pressures, known as transmembrane pressure, causes water to flow, by convection, from the blood into the dialysing fluid.
Successful dialysis treatment depends on accurately controlling the amount of water withdrawn from the blood, falure to do so possibly resulting in the patient manifesting negative and, at times, even critical reactions, such as cardiocirculatory arrest. Furthermore, water withdrawal should be programmed so as to vary appropriately from one patient to another.
Standard dialysers as of present comprise membranes having a relatively low permeability coefficient and, as such, require relatively high transmembrane pressures for withdrawing the required amount of water from the patient's blood. Withdrawal is performed directly by the operator, who checks the weight of the patient lying on a bed fitted with a scale, and adjusts the transmembrane pressure, while at the same time observing the effect of the adjustment on the patient's scale. On such dialysers, transmembrane pressures of around 150-300 mmHg produce no considerable variation in water withdrawal rate, in that, small spontaneous changes in blood pressure, caused, for example, by changes in blood flow, and similar unpredictable changes in the negative pressure of the dialysing fluid, caused, for example, by air bubbles, are both of about 20-30 mmHg/hour. Though the operator may, of course, make the necessary adjustments to transmembrane pressure, by observing the weight of the patient, say, once every half hour or once an hour, drawing off a preset quantity of water per hour is rendered difficult for the reasons already expounded.
Increasing use is currently being made of dialysers comprising membranes with relatively high permeability coefficients, and providing for more efficient and faster cleansing of the blood. Such dialysers, however, have rendered manual control by the operator both difficult and dangerous, by enabling water to be withdrawn at transmembrane pressures of around 20-50 mmHg. As such, normal changes in blood pressure and a rise or fall on the dialyser are sufficient for varying the water withdrawal rate in such a manner as to be intolerable by the patient. An operator must therefore be stationed permanently beside the patient, to prevent sharp changes in the patient's weight from producing the negative reactions already mentioned.
To overcome this drawback, which limits the employment of highly permeable dialysers, control devices are currently used for directly determining the patient's water balance, i.e. the difference between the amount of clean dialysing fluid fed into the dialyser and the amount coming out, which equals the amount fed in plus the amount of water withdrawn from the blood. Currently employed control devices are based on volumetric principles and present two distinct drawbacks. Firstly, volumetric control is affected by the presence of air bubbles, the volume of which depends on the pressure involved. Secondly, the fluid coming out of the dialyser, which is obviously contaminated, is fed into a graduated vessel enabling the operator to assess the amount of fluid withdrawn from the patient. This obviously creates a serious hygiene problem, besides conflicting with strict safety regulations whereby the said contaminated fluid must be fed directly into a drainage network, for preventing anyone from even accidentally coming into contact with it.